1. Field
The present disclosure relates to optical receivers. More specifically, the present disclosure relates to optical receivers in which inputs to a transimpedance amplifier can be isolated during a calibration mode.
2. Related Art
As illustrated in FIG. 1, a typical existing optical receiver includes an optical-to-electrical (OE) converter that receives an optical signal, and which generates a corresponding electrical signal. Then, an amplification chain converts the electrical signal into digital voltage levels.
For example, a common OE converter is a photodetector (PD). It converts an incoming optical signal to a current (iRD). This current is converted to a voltage by a transimpedance amplifier (TIA). In turn, the small voltage swing from the TIA's output (V0) is converted into digital signal levels (digital out) through several stages of amplification (amp. chain). Note that the modular integration of a PD on a separate substrate from the CMOS modules using an electrical interconnect usually introduces a large parasitic capacitance (CPD in FIGS. 2A and 2B) that can limit the operating bandwidth of the optical receiver.
In order to properly set the operating conditions of the electrical amplification stages during a calibration mode, it is often useful to isolate the TIA from the input current, iPD. As shown in FIGS. 2A and 2B, in an existing optical receiver, this isolation can be achieved by inserting a switch along the path from the PD to the TIA. When the switch becomes opaque, no current flows through the switch, and as a consequence the TIA sees no input current, henceforth referred to as the ‘zero-current input condition’. However, this isolation technique has several drawbacks. Notably, the performance of the optical receiver may be degraded by the added parasitic resistance and capacitance associated with the introduction of such a switch.
Consider the path from the PD to the TIA as an electrical node. The time constant at which this node can charge and discharge is determined by the impedance seen at this node, which is set by the capacitance and the resistance of this node to ground. In particular, the RC time constant is the product of the dominant capacitance, CPD, and the resistance seen by this capacitor. Note that large capacitance and large resistance values imply a large RC time constant, and a large RC time constant implies that the optical receiver responds slowly to changing input data.
Given a predefined value of CPD, the equivalent resistance seen at this node is constrained by a frequency-response target (i.e., a desired RC time constant). Therefore, based on the technology, power limitations, and noise considerations, the equivalent input impedance of the TIA may have a narrow range of acceptable values. In this range, the additional series resistance associated with the introduction of an isolation switch between the PD and the TIA (as shown in FIGS. 2A and 2B) may significantly deteriorate the performance of the optical receiver.
The degradation in the frequency response of the optical receiver through the introduction of a series resistance is illustrated using the simplified circuit shown in FIG. 3B (which corresponds to the existing optical receiver shown in FIG. 3A). Denoting the parasitic capacitance CPD in this circuit as Clarge and the TIA's equivalent input impedance (RTIA) as Rsmall, which is approximately the resistance Rf in the TIA's feedback path divided by the TIA's gain A. Note that the qualifiers ‘large’ and ‘small’ indicate the relative values of these components from a designer's perspective. In general, Clarge is 50 to 100× larger than the typical logic gate capacitance, and Rsmall is 10 to 20× smaller than the typical ‘on’ logic gate resistance.
As shown in FIGS. 4A and 4B, from a resistance perspective, if a switch is added between the TIA and the PD, the RC time constant increases to Clarge·(Rsmall+Rswitch), where Rswitch is the equivalent resistance of the switch. If Rswitch is large compared to Rsmall, Rswitch can significantly degrade the frequency response. To lower Rswitch, the width of the switch can be increased, but increasing the size of the switch adds additional parasitic capacitance so that the RC time constant may still be degraded. A more complete expression for the RC time constant includes this effect, i.e., (Clarge+Cswitch)·(Rsmall+Rswitch), where Cswitch is the equivalent capacitance of the switch. Consequently, because there is a tradeoff between Rswitch and Cswitch, there is an optimal size for the switch which minimizes the RC time constant. But even at this minimum value, the degradation in the frequency response of the optical receiver may not be acceptable.
Another technique that is used to provide DC isolation and to allow for independent DC biasing of the TIA (so that the operating condition can be set at a desirable point) includes AC coupling the PD to the input of the TIA. For example, the PD may be capacitively coupled to the input of the TIA. However, because of this AC coupling, data communicated to the optical receiver by an optical transmitter may need to be DC balanced, and may also need to have enough transitions so that the time between transitions is small compared with the relevant time constants at the optical receiver. These requirements may increase the overhead with a commensurate degradation in the effective data rate.
Alternatively, the output from the TIA can be averaged using an RC filter. The average of the output may be used as a common-mode reference for iPD, i.e., it can provide a fixed DC input to the TIA. However, this technique does not provide true isolation, and may also require that DC-balanced data be communicated to the optical receiver by an optical transmitter. In addition, the averaging may introduce additional problems, including: residual ripple (i.e., the averaging may not be perfect); the averaging circuit may contribute undesirable parasitics along the signal path; and the averaging may behave poorly when the input signal is saturated.
Hence, what is needed is an optical receiver without the above-described problems.